Metal vapor lamps frequently include an elongated alumina or quartz arc tube with an electrode positioned at each end thereof. A metal fill such as mercury, sodium, cadmium, thallium or zinc and usually a rare gas such as Xenon at a pressure in the range of about 14-30 torr is provided within the arc tube. Normally, the metal additive is heated to effect vaporization and provide the radiation species for the lamp. This arc tube is sealed within an outer envelope which is, in turn, formed for connection to a voltage source by way of some form of ballast means. A preferred form of metal vapor lamp is known as a high pressure sodium lamp and includes a mercury and sodium fill therein. Examples of known high pressure sodium lamps are set forth in U.S. Pat. Nos. 4,445,073; 3,900,753 and 4,179,640.
High pressure sodium lamps have become commercially useful within the past few years because of their relatively high efficiency and improved color rendering as compared with the monochromatic yellow light associated with low pressure sodium lamps. These high pressure sodium lamps are distinguishable from the better known low pressure sodium lamps because of the increased operating pressure which may be anywhere from several to about 1000 millimeter Hg.
A major problem associated with high pressure sodium lamps is the difficulty of starting such lamps and more particularly the undesired need for a relatively high potential source in order to effect the desired starting. One means often employed to overcome this undesired need for increased source potentials is the utilization of a so-called "starting aid". Examples of high pressure discharge lamps employing a starting aid include U.S. Pat. Nos. 4,445,073; 3,900,753 and 3,721,846 all assigned to the present Assignee. However, it has been found that there are instances wherein the added starting capability provided by a starting aid is not sufficient to effect the desired starting of the lamp without also undesirably increasing the potential available from a potential source. Obviously, increasing potentials is not always feasible and is certainly undesirable.
Other examples of high pressure sodium lamps utilizing starting aides in an effort to reduce the required starting voltage include U.S. Pat. No. 4,316,122 to Yamazaki et al; an article entitled "Developments In High Pressure Sodium Discharge Lamps" by J. A. J. M. van Vliet and C. A. J. Jacobs distributed at the CIBS National Lighting Conference 1980 and an article entitled "Investigation On Built-In Starter Unit For High Pressure Sodium Lamps" by Naoki Saito and Hiroshi Gion appearing in the National Technical Report Vol. 27 No. 3 June 1981 at the Lighting Division, Matsushita Electronics Corp.
As set forth in the patent of Yamazaki et al (122), the lowest starting voltage obtainable was about 1.3 KV without an electrode extension and a very low 950-volts when an extension was added to the electrode. Also, FIG. 6 of the article of van Vliet and Jacobs indicates a starting voltage in the range of 1.3 to 1.4 KV at a pressure of about 114 torr in an HPS lamp. Moreover, the article of Saito and Gion, FIG. 2 lower curve, indicates a starting voltage of about 1.1 KV when a starting aid is employed in known high pressure sodium lamps. Thus, the above-mentioned patent and published articles suggest that a voltage of not less than about 950 volts is required to effect the desired operation of a high pressure sodium lamp.
Another known technique for improving the starting capability of high pressure sodium lamps is the utilization of a so-called "Penning" gas mixture in conjunction with a starting aid and an increased arc tube diameter. However, it has been found that the improved starting capability provided by the use of the "Penning" gas method is offset by the reduced efficiency and reduced life of the resulting lamps. Examples of such structures includes U.S. Pat. Nos. 3,900,753 of Richardson and 4,037,129 of Zack.
Additionally, it is known to include a supplemental high voltage generating means to a high pressure sodium lamp and accompanying ballast circuitry. Such a configuration is set forth in U.S. Ser. No. 456,679 filed Jan. 10, 1983 entitled HID Lamp Starting Apparatus and assigned to the Assignee of the present application. However, as set forth therein, starting voltages in excess of about 1000 volts were required to effect the desired starting of the discharge lamp.